I. Field of the Invention
This invention relates to a high performance overlay of the metallic surfaces of rock bit components such as rotary cones, rock bit legs supporting the cones and the exposed surfaces surrounding the cutters mounted within the face of a drag type rock bit.
More particularly, this invention relates to the application of a high performance overlay or coating to the exposed surfaces of steel rotary cones and their supporting legs of rotary cone rock bits. The overlay coating also has an application for the cutting face surrounding diamond cutters mounted within the face of diamond drag rock bits and the like.
II. Background
Hardfacing of rock drilling bit cones for the purpose of inhibiting cone erosion and wear during known harsh rock drilling conditions has been done before with varying degrees of success.
For example, U.S. Pat. Nos. 4,708,752 and 4,781,770 teach the use of lasers to either harden the surface of the rotary cones of a rock bit or entrain a stream of hardfacing material into the laser beam to apply a layer of hardfacing material to the surface of the rotary cones. Both of the foregoing patents are assigned to one of the assignees of the present invention and are incorporated herein by reference.
U.S. Pat. No. 4,685,359 describes a method of manufacturing a steel bodied bit in which a hardfacing of a highly conformable metal cloth containing hard, wear resistant particles is applied to rock bit faces and to the interior of nozzle openings and the like. The cloth known as "CONFORMA CLAD", manufactured by Imperial Clevite, Inc. of Salem, Ind., must first be cut to shape to fit the component to be hardfaced prior to brazing the cloth to the workpiece; a time consuming and difficult process. This method is disadvantaged in that the cloth material, when it is metallurgically attached to the workpiece in a furnace, changes the physical properties of the base material to the detriment of the finished product.
U.S. Pat. No. 5,279,374 describes a method of forming an erosion resistant hard refractory metal coating on a roller bit cone. This patent teaches the method of thermally spraying fine (10 to 33 microns) tungsten carbide powder mixed with 8 to 15 percent by weight cobalt binder powder to form a continuous layer on the outer surfaces of the cone and the sintered carbide drilling inserts entrained thereon. This patent teaches that the adherence of the coating is dependent on the penetration of the metallic matrix of the insert by coating material. While the foregoing method does somewhat inhibit bit cone erosion, it has some serious disadvantages. Any crack initiated in the brittle coating on a carbide insert tends to propagate into the carbide insert substrate reducing the breakage resistance of the insert thereby shortening bit life. Any layer of carbide of significant thickness penetrating the carbide inserts in a bit cone changes the geometry of the inserts making them more blunt. This materially reduces the ability of the inserts to penetrate the rock, thereby reducing the drilling rate of the bit.
The inventors of the present invention have performed extensive field testing of a new type of coating applied to the steel cones of roller type drill bits. This coating is named "Armcore-M" and was developed by Amorphous Technologies International (AMTECH). Armcore-M is basically a mixture of iron, chromium and cobalt powders developed for abrasion and erosion resistance and is covered by U.S. Pat. No. 4,725,512. This coating is applied to a steel substrate using a thermal spray welding technique and it is considered to undergo transformation hardening when stressed or forced into deformation. The results of the aforesaid tests were disappointing in that they were not significantly better in erosion and abrasion resistance than commercially available normal low velocity thermal spray coatings. Even though the Armcore-M coating itself is fairly wear resistant, it does not compare favorably to the ultra high velocity Super D-Gun process coating of the present invention because of its low bond strength to the steel substrate.
U.S. Pat. Nos. 4,826,734 and 5,075,129 describe the basic detonation gun technology and are incorporated herein by reference. In particular, U.S. Pat. No. 4,826,734 teaches the Super D-Gun.TM. process whereby the overlays produced on rock bit surfaces of the present invention have a hardness of at least 900 Kg/m.sup.2 VHN, a strain-to-fracture of about 6.0 mils/inch (0.006") and bond strength that greatly exceed the standard ASTM 633 test strength of about 10,000 PSI.
The Super D-Gun process overlay of the present invention has a nominal composition of 83 weight % tungsten, 14 weight % cobalt and 3 weight % carbon. To achieve the above properties of the coatings of the present invention, it is necessary to accelerate the particulates in the Super D-Gun process to about 3,000 ft/sec. (or greater).
The present invention, using an improved ultra high particle velocity (in excess of 3,000 ft/second) detonation gun thermal spray equipment, produces a monolithic carbide coating that is very strongly adhered to the steel cone surfaces. This includes the areas around and proximate the carbide inserts. The carbide/cobalt spray does not adhere to the carbide inserts because the particles used for coating are much larger than the mean free path of the cobalt binder of the inserts and do not penetrate the binder. But the ultra high velocity of the carbide particles (in excess of 3,000 ft/second) impinging on the protruding carbide inserts does significantly increase the compressive strength of the inserts. It is believed that an effect similar to shot peening induces a significant residual compressive stress in the insert surfaces thereby enhancing the fatigue properties of the inserts.
An improved method of overlaying or coating of rock bit cones and the like is disclosed that incorporates advanced coating materials and application methods.